Stainless Steel Coil Tube — Chemical Degreasing of Inner Surface (Alkaline Circulation Flushing)
1. Product Overview
The Stainless Steel Coil Tube (Coil Tube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubing product supplied in coiled configuration for downstream forming, bending, and fabrication operations. A critical stage in the post-manufacturing treatment of this product is the Chemical Degreasing (化学脱脂) of the inner surface, executed via an Alkaline Cleaning Agent Circulation Flushing (碱性清洗剂循环冲洗) process. This procedure ensures the complete removal of organic contaminants — including rolling oils (轧制油), drawing oils (拉伸油), emulsions, and other hydrocarbon residues — that are inevitably introduced during cold-rolling, cold-drawing, and forming operations. The result is a chemically clean, oxide-free interior surface that serves as the essential prerequisite for subsequent pickling, passivation, electropolishing, or direct industrial service.
2. Material & Properties
- Base Material: Austenitic stainless steel grades (e.g., AISI 304 / 304L / 316L), cold-drawn or cold-rolled into seamless coil tube geometry.
- Inner Surface Finish (Post-Degreasing): Free of visible oil films, emulsions, and organic residues; surface energy restored to a clean metallic state suitable for wet chemical treatment.
- Corrosion Resistance: Maintained and enhanced by ensuring no residual chlorides or carbonates from the degreasing cycle remain on the surface.
- Mechanical Properties: Unaffected by the degreasing process; yield strength, tensile strength, and elongation retain their as-drawn specifications.
- Chemical Compatibility: The alkaline cleaning agent (typically NaOH-based or Na₃PO₄-based, pH 12–14) is selected to avoid pitting or intergranular attack on the stainless steel substrate.
3. Specifications
| Parameter | Specification |
|---|---|
| Process Type | Alkaline Cleaning Agent Circulation Flushing (碱性清洗剂循环冲洗) |
| Target Surface | Inner Surface (内表面) |
| Cleaning Agent Type | Alkaline aqueous solution (NaOH / Na₃PO₄ / surfactant blend) |
| Solution pH | 12.0 – 14.0 |
| Operating Temperature | 60 °C – 80 °C (140 °F – 176 °F) |
| Cycle Time (per pass) | 15 – 45 minutes (adjustable per tube geometry and contamination level) |
| Flow Configuration | Closed-loop circulation with spray nozzles directed into tube bore |
| Residual Oil Removal Efficiency | ≥ 99.5% (verified by gravimetric or infrared spectroscopic testing) |
| Post-Clean Surface Tension | ≥ 40 mN/m (indicating hydrophilic, clean surface) |
| Contaminants Removed | Rolling oil (轧制油), drawing oil (拉伸油), emulsions, waxes, grease, hydrocarbon residues |
| Applicable Tube OD | 3 mm – 50 mm (typical range; scalable) |
| Applicable Tube Wall Thickness | 0.3 mm – 3.0 mm |
| Post-Process Rinsing | Deionized water (DI water) circulation rinse, followed by air drying |
4. Manufacturing Process
4.1 Process Flow
- Tube Loading: The stainless steel coil tube is fed into the degreasing chamber where both ends are sealed with high-temperature resistant gaskets to establish a closed fluid circuit.
- Agent Injection & Circulation: The pre-heated alkaline cleaning agent (pH 12–14, 60–80 °C) is injected into the tube bore via a high-pressure pump. Internal spray nozzles ensure uniform coverage of the entire inner surface, even at bends and tight radii.
- Solubilization & Emulsification: The alkaline environment saponifies ester-based rolling and drawing oils, while non-ionic and anionic surfactants emulsify non-saponifiable hydrocarbons, converting adherent films into a stable aqueous dispersion.
- Circulation Duration: The solution is maintained in continuous circulation for 15–45 minutes. Turbulent flow conditions (Reynolds number > 4000) ensure mechanical shear assists chemical detachment from the tube wall.
- Agent Recovery & Filtration: The spent cleaning solution is extracted, filtered to remove emulsified oil particles, and either regenerated (via centrifugation or coalescer separation) or replaced according to contamination threshold.
- Intermediate Rinse: Warm water (40–50 °C) circulation rinse removes residual alkaline solution and dissolved contaminants.
- Final Rinse: Deionized water (conductivity < 10 µS/cm) circulation rinse eliminates dissolved salts and ensures no alkaline carryover remains.
- Drying: Compressed air blow-through or forced convection drying removes residual moisture, preventing water spot formation.
- Quality Verification: Surface cleanliness is confirmed via water break test, contact angle measurement, or infrared spectroscopy for residual carbon content.
4.2 Critical Process Parameters
- Temperature Control: Maintaining 60–80 °C is critical — below 60 °C, saponification kinetics are too slow for complete oil removal; above 80 °C, risk of alkaline corrosion and agent decomposition increases.
- pH Stability: Continuous pH monitoring ensures the cleaning agent remains within the effective range throughout the batch cycle.
- Flow Rate: Optimized to achieve Reynolds numbers exceeding 4000 within the tube bore, ensuring turbulent conditions that mechanically dislodge stubborn oil films.
- Agent Concentration: Typically 3–8% active alkaline solids, supplemented with 0.5–2% surfactant and corrosion inhibitors (e.g., silicates, borates) to protect the stainless steel substrate.
5. Applications
The chemically degreased inner surface of the stainless steel coil tube is essential for the following downstream applications:
- Pharmaceutical & Bioprocess Equipment: Hygienic piping where zero organic residue is mandated (FDA / EU GMP compliance).
- Food & Beverage Processing: CIP (Clean-In-Place) compatible tubing requiring a pristine inner surface for sanitary fluid transfer.
- Semiconductor & Ultra-Pure Water Systems: Tubing where trace organic contamination must be eliminated to maintain ultra-low total organic carbon (TOC) levels.
- Chemical & Petrochemical Processing: Corrosion-resistant piping where surface cleanliness prevents localized corrosion initiation.
- Medical Devices & Endoscopes: Precision tubing requiring biocompatible, residue-free internal surfaces.
- Subsequent Surface Treatments: Prepares the surface for pickling and passivation, electropolishing, or PVD/CVD coating processes that require chemically clean substrates for optimal adhesion.
6. Quality Certifications & Standards
- ISO 9001:2015 — Quality Management System certification ensuring process consistency and traceability.
- ISO 14001:2015 — Environmental Management System, governing responsible handling and disposal of alkaline cleaning agents.
- ASTM A269 / A270 — Standard specifications for seamless austenitic stainless steel tubing, including surface condition requirements.
- ASTM A773 — Specification for wrought austenitic stainless steel weldless tube (dimensions and mechanical properties).
- EN 10216-5 — European standard for seamless stainless steel tubes for pressure purposes.
- 3-A Sanitary Standards / FDA 21 CFR Part 117 — Applicable where tubing is used in food and pharmaceutical service.
- Surface Cleanliness Verification: Compliant with ASTM A967 (passivation verification) and internal visual/spectroscopic inspection protocols.
7. Unique Selling Points
Environmentally Responsible: The closed-loop circulation system maximizes cleaning agent utilization, reducing wastewater volume by up to 60% compared to batch immersion methods, while enabling agent regeneration and reuse.
Superior Contaminant Removal: The combination of high-pH alkaline chemistry, elevated temperature, and turbulent internal flow achieves ≥ 99.5% removal of rolling oil, drawing oil, and emulsified hydrocarbon residues — exceeding industry benchmarks.
Material-Safe: The alkaline formulation incorporates corrosion inhibitors (silicates, borates) that protect the austenitic stainless steel substrate, eliminating the risk of stress corrosion cracking or intergranular attack associated with aggressive acid-based cleaning.
Process Scalability: The circulation flushing methodology is adaptable to tube diameters from 3 mm to 50 mm and accommodates both straight and coiled configurations, offering flexibility for diverse production volumes.
Seamless Integration: Designed as a dedicated pre-treatment stage that feeds directly into downstream pickling, passivation, or electropolishing lines, minimizing handling and recontamination risk.
8. Technical Notes & Recommendations
- Post-degreasing, the tube should proceed to pickling and passivation within 48 hours to prevent atmospheric recontamination.
- For tubes containing residual emulsions resistant to standard alkaline treatment, a two-stage process (alkaline degreasing followed by solvent vapor degreasing) may be recommended.
- All cleaning agent waste streams must be treated in accordance with local environmental regulations prior to discharge.
- Regular calibration of pH meters, temperature sensors, and flow meters is essential to maintain process consistency and product quality.